1. Field of the Invention
The present invention relates to photolithography. More particularly, the present invention relates to a projection lens of an exposure apparatus of a photolithography system, and to the associated phenomenon of flare that causes defects in a pattern formed on a wafer by light focused on the wafer by the projection lens.
2. Description of the Related Art
In general, flare is a phenomenon that produces a bad exposure in a photolithographic process due to defects of a projection lens of the exposure apparatus of the photolithography system. More specifically, when a portion of the surface of the lens is defective, the exposure light is dispersed at the defective portions, and photoresist patterns are formed incorrectly by the exposure light. Here, the defects at the surface of the lens which may produce flare-include contaminants, scratches, or a difference in refractory indices between portions of the lens. Light passing through such defective portions of the lens during exposure scatters and thus, the light does not focus properly on the photoresist layer.
The flare phenomenon will be described more fully with reference to FIGS. 1 and 2. Referring to FIGS. 1A and 1B, an exposure apparatus for performing a photolithographic process includes a lens 14 for scaling down and projecting a light shielding pattern 11 of a mask 10 onto a predetermined portion of a wafer 12. The lens 14 is interposed between the mask 10 and the wafer 12. The top surface of the wafer 12 is coated with a photoresist layer (not shown).
As shown in FIG. 1A, if no defect occurs at the surface of the lens 14, the shielding pattern 11 on the mask 10 is projected on a reduced scale onto the photoresist layer. Accordingly, photoresist patterns 12a are formed on the wafer 12.
On the other hand, as shown in FIG. 1B, if defects occur at the surface of the lens 14, light disperses at the defective portions 15 of the lens 14. The dispersion of light results in an irregular distribution of light on the photoresist layer during the exposure, and decreases the contrast of the image. In addition, portions of the wafer 12 corresponding to and adjacent to the defective portions 15 may become excessively exposed. As a result, an on chip variation phenomenon occurs in which photoresist patterns 12b on the wafer 12 are deformed, or the widths of photoresist patterns 12b formed in one field vary. As the photolithographic process is repeated, the lens 14 of the exposure apparatus becomes more severely defective, and the amount of flare varies.
Accordingly, in photolithography, the amount of flare of a lens and the position on the wafer which is affected by flare must be measured and determined for every exposure process if the photoresist patterns are to be formed as desired.
However, conventional photolithography systems do not have tools for identifying whether flare is produced by a projection lens, for identifying whether a wafer is affected by flare, and/or for determining the extent of a flare-affected region on a wafer. Thus, it is difficult to correct for the flare, i.e., to avoid bad exposures.
An object of the present is to solve the above-described problems related to flare in the photolithography process.
More specifically, it is a first object of the present invention to provide a mask which is capable of being used in a photolithography system to identify whether flare is being produced by the projection lens of the system and to quantify the amount of flare.
It is a second object of the present invention to provide a method of manufacturing such a mask.
It is a third object of the present invention to provide a method of identifying a flare-affected region on a wafer.
It is a fourth object of the present invention to provide a method of correcting for the flare to produce the photoresist patterns having desired line widths in a region that would otherwise be affected by the flare.
It is a fifth object of the present invention to provide a method of designing the line width of transmission patterns (or shielding patterns) of a mask to compensate for the flare produced by the lens of the photolithography system with which the mask is to be used.
The mask according to the present invention includes a mask substrate having a light shielding region and a light transmission region, and a plurality of alternating line and space patterns formed in each of the light shielding region and the light transmission region. The line and space patterns formed in the light shielding region correspond to the line and space patterns formed in the light transmission region.
The plurality of line patterns all have the same line width, and the plurality of space patterns all have the same line width. The space patterns may be light transmission patterns in the form of grooves interposed between the line patterns.
A main light shielding layer formed on a predetermined portion of the mask substrate may divide the mask substrate into the light shielding region and the light transmission region. The main light shielding layer defines at least one group (row, for example) of light transmission patterns. At least one sub light shielding layer is formed in the light transmission region of the mask substrate, and defines at least one group of light transmission patterns corresponding to those defined by the main light shielding layer.
A plurality of rows of the light transmission patterns are formed in the main light shielding layer as spaced from one another in a longitudinal direction of the mask. Likewise, a plurality of rows of light transmission patterns are formed in the at least one sub light shielding layer as spaced form one another in the longitudinal direction.
The light transmission patterns all have the same size. In addition, the (latitudinal) spacing of the light transmission patterns in the rows thereof is uniform, and the longitudinal spacing of the rows of the light transmission patterns is also uniform.
The boundary between the main light shielding layer and the light transmission region is preferably linear and extends at a right angle to a line passing through the centers of longitudinally aligned ones of the light transmission patterns. The light shielding region and the light transmission region may have the same size, and the light transmission patterns are located along a longitudinal area of the mask substrate.
In order to manufacture the mask, a light shielding layer of a light-blocking material is first formed on the mask substrate (transparent). At least one light transmission pattern is formed in the light shielding layer. Subsequently, the main light shielding layer is formed by removing one portion of the light shielding layer from the light transmission region, and the at least one sub light shielding layer is formed by leaving a portion of the light shielding layer in the light transmission region around the light transmission patterns.
In forming the light transmission pattern and forming the main light shielding layer, portions of the main light shielding layer may be removed such that the light transmission patterns join the boundary between the main light shielding layer and the light transmission region.
In order to identify a flare-affected region on a wafer, photoresist patterns are formed on the wafer using the mask to carry out a photolithography process. The line widths of photoresist patterns formed by the line and space patterns of the light shielding region of the mask, and the line widths of the photoresist patterns formed by the line and space patterns of the light transmission region of the mask are measured. Next, the line widths of the photoresist patterns are compared. If the difference in the line widths of the photoresist patterns differ by more than a predetermined value, such as the calibrated precision of the measuring apparatus, it is determined that the projection lens is producing flare. The amount of flare is calculated based on the difference in line widths between the photoresist patterns formed by light from the light shielding region of the mask and the photoresist patterns formed by light from the light transmission region of the mask.
Next, an interval over which the line widths of the photoresist patterns vary considerably in a region corresponding to the boundary between the light shielding region and the light transmission region of the mask is discerned. Then, a circular region of the wafer, having a radius equal to the interval is estimated as having been affected by the flare. The circular region is deemed to be centered at a position corresponding to the boundary between the light transmission region and the light shielding region.
Once the flare-affected region of the test wafer is identified, it can be analyzed to manufacture a new mask that will correct for the flare produced by the projection lens. To this end, the effective amount of flare in the flare-affected region is calculated. Then, the light transmission patterns of the new mask are designed, i.e., configured, based on the effective amount of flare such that when the new mask is used in the photolithography system, the photoresist patterns will have the desired line widths despite the flare produced by the projection lens. In particular, the line widths of light transmission patterns formed in a mask region corresponding to the flare-affected region and the spacing between the light transmission patterns are set up in the new mask taking the effective amount of flare into consideration.
In measuring the effective amount of flare in the flare-affected region, the open ratio of the flare-affected region is determined, and the amount of flare in the flare-affected region is calculated by converting the amount of flare influencing all of the entire exposed regions of the wafer into the open ratio of the flare-affected region.
In order to ensure an even greater accuracy in the design of the new mask, the region on the test wafer on which the photoresist patterns are formed is discriminated into a plurality of mesh regions constituting a matrix. Convolution values with respect to the line widths of the photoresist patterns in each mesh region are calculated. An error value of the critical dimension (CD) of the photoresist patterns in each mesh region is calculated using the following Equation:       CD    ⁢          xe2x80x83        ⁢    error    ⁢          xe2x80x83        ⁢    wafer    =                    convolution        ⁢                  xe2x80x83                ⁢        value                    convolution        ⁢                  xe2x80x83                ⁢        max              xc3x97    Max    ⁢          xe2x80x83        ⁢    CD    ⁢          xe2x80x83        ⁢    error  
In this equation, xe2x80x9cCD error waferxe2x80x9d represents an error value of the CD of the photoresist patterns in each mesh region on a corresponding wafer, and xe2x80x9cconvolution valuexe2x80x9d represents the convolution value with respect to the line widths of the photoresist patterns in each mesh region on the corresponding wafer, and xe2x80x9cconvolution maxxe2x80x9d represents the maximum value of all of convolution values with respect to the mesh regions, and xe2x80x9cMax CD errorxe2x80x9d represents the maximum difference in the line widths of the photoresist patterns.
Then, the error value of the CD is factored into the design of the patterns in a region of the new mask corresponding to the mesh region.
Still further, an error value of critical dimension (CD) of the photoresist patterns in each mesh region is calculated using the following Equation 1. A mask error enhancement factor (MEEF) of the reduction projection exposure apparatus of the photolithography system for forming the photoresist patterns is calculated. The error value of the CD of patterns in the mask is measured using the following Equation 2. The CD of the patterns of the new mask is designed based on the error value of the CD of the patterns of the original mask.                     [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            ⁢              :            ⁢              xe2x80x83            ⁢      CD      ⁢              xe2x80x83            ⁢      error      ⁢              xe2x80x83            ⁢      wafer        =                                        convolution            ⁢                          xe2x80x83                        ⁢            value                                convolution            ⁢                          xe2x80x83                        ⁢            max                          xc3x97        Max        ⁢                  xe2x80x83                ⁢        CD        ⁢                  xe2x80x83                ⁢                  error          ⁢                      
                    [                      Equation            ⁢                          xe2x80x83                        ⁢            2                    ]                ⁢                  :                ⁢                  xe2x80x83                ⁢        CD        ⁢                  xe2x80x83                ⁢        correction        ⁢                  xe2x80x83                ⁢        mask            =                        Mag          xc3x97          CD          ⁢                      xe2x80x83                    ⁢          error          ⁢                      xe2x80x83                    ⁢          wafer                MEEF              ⁢      xe2x80x83  
In Equation 2, xe2x80x9cMagxe2x80x9d represents a reduction projection multiple of the exposure apparatus.